1. Field of the Invention
This invention relates to a mirror device for a camera having a first mirror arrangement flippable from a finder-viewing position to an exposure allowing or retracted position and a second mirror arrangement flippable relative to this or first mirror arrangement.
2. Description of the Prior Art
Recently, there have been proposed single lens reflex cameras in which a first mirror that partly permeates light is arranged to pivotally move from the finder-viewing position to the light path-to-film clearing position and a second mirror is mounted on the first mirror and arranged to be pivotable from a position where the light permeating from the first mirror in the finder is directed to a photosensitive element for light metering or range finding.
In the single lens reflex cameras of this kind, one of the prior known methods for determining the terminal ends of pivotal movement of the second mirror is that as described in U.S. Pat. No. 4,293,209, wherein the second mirror is urged by a spring to open relative to the first mirror and further a camming surface is formed on the framework of the camera, whereby the second mirror is brought into, and always maintained in, sliding movement on the camming surface, so that the opening angle of the second mirror in the finder-viewing position of the first mirror is determined, and when in the non-viewing position, the second mirror is held against the bias force of the spring. However, as is well known, the accuracy with which each mirror is located in the ends of pivotal movement must be very high. The use of only one camming surface for defining the opening and closing positions of the second mirror as in the aforesaid prior known proposal, therefore, leads to difficulty in making adjustment against the tolerances for the dimensions and the location of the parts. In more detail, when the angular position of the second mirror relative to that camming surface has been adjusted, for example, in the opening end, it is at the closing end that a location error takes place. If this error were corrected, the once-established adjustment of the opening position would be broken. In order to allow for both of the positions to be adjusted with high accuracy as it should be in actual practice, the necessity of using an additional mechanism of complex structure arises. Another problem is that the adjusting operation in the assembly line becomes troublesome and time-consuming.
Yet another problem of the aforesaid prior known proposal is that as the first mirror moves toward the nonviewing position with increasing speeds, when it happens that the prescribed point is overrun slightly by inertia, that camming surface urges the second mirror also to move beyond the limit of a range of movement thereof with exertion of more forces than necessary for holding it in the closing position, because these forces deform the second mirror. To avoid this, there is a need to add surplus means of complicated structure, affecting the production cost and the availability of space.
Another example of a proposed method is in U.S. Pat. No. 4,320,945 wherein use is made of a toggle mechanism, whereby the direction to which the second mirror is urged is changed at an intermediate point during the pivotal movement of the first mirror. That is, as the first mirror pivots toward the non-viewing position, after it passes that point, the force of the drive spring acts on the second mirror in a direction to close. Conversely when moving to the viewing position, the force changes its direction at that intermediate point. From this time onward, the second mirror is driven to move away from the first one. And, according to this proposal, what is necessary for locating the second mirror when in the viewing position, is only a pin fixedly secured to the framework of the camera. Besides this, the mirror apparatus is necessarily provided with means for causing the second mirror to follow up the first mirror until the prescribed point as the first mirror pivots in either direction.
However, in the second proposal, because the spring for the second mirror changes over between pushing and pulling at an intermediate point during the opening or closing operation, there is produced a problem that no smooth mirror operation is obtained.
Also, in the second proposal, the spring of the second mirror must be connected at the opposite end to the framework of the camera. Therefore, the step of installing the mirror device within the interior of the camera housing must be followed by a step of mounting that spring between the second mirror support and the framework of the camera housing, giving rise to a problem in that the efficiency of the assembly line is reduced remarkably.
Another problem is that because that spring must be constructed in the form of the bias spring of the toggle mechanism, a spring with a long leg is selected for employment as that spring, worsening the space efficiency. This becomes serious when the bulk and size of the camera is to be minimized.